Normal fetal growth is a critical component to a healthy pregnancy and the long-term health of the child. Pivotal to understanding the dynamics of human fetal growth and to define normal and abnormal fetal growth is development of standards for fetal anthropometric parameters measured longitudinally throughout gestation and which can be used to develop interval velocity curves and customized for genetic and physiological factors. We proposes to conduct a multicenter prospective observational study (1) to establish a U.S. national standard for normal fetal growth (velocity) and size for gestational age;(2) to create an individualized standard for optimal fetal growth;and (3) to improve accuracy of fetal weight estimation. This study will recruit 2,400 healthy, non-obese, low risk pregnant women from 4 clinical centers. Approximately 600 women each will come from the following self-identified race/ethnicity background: African American, Asian, Caucasian, and Hispanics. Women will be recruited in the first trimester and followed up through pregnancy. In addition, 600 obese women will also be recruited. Each woman will receive 5 scheduled ultrasound exams, which include 2-dimensional and 3-dimentinoal fetal anthropometric measurements. Uterine artery and fetal Doppler studies will be performed at selected gestational weeks. After delivery, neonatal anthropometric measures will be carefully taken. Obstetric and neonatal information will be extracted from medical charts. The national standard for normal fetal growth will have important implications in clinical practice and research.